So-called bilevel lighting systems have been in use at least since the early 1970's. Earlier systems involved dimming devices which were preset at a desired reduced level of power and then the control was manually switched between full power and the pre-selected level of reduced power. A later refinement involved more automatic control and took ambient light level into account. Various patents and trade literature relating to bilevel lighting systems are discussed below.
The 1990 Wide-Lite Buyer's Guide depicts a bilevel lighting system. In such system, switching between power levels is by a two-wire control circuit. Such Guide indicates that luminaires can be provided with factory-installed ballasts and switching devices; for retrofits, a remote ballast and, presumably, a remote switching device are used. And the ballast and switching device are described as being in a housing "for heat transfer." The photos accompanying the description suggest that for a luminaire-mounted ballast and switching device, the mentioned housing is separate from and in addition to any housing which is part of the luminaire. There is also indication in the Wide-Lite Buyer's Guide that the ballast used in bilevel lighting systems differs in some way from ballasts used in conventional "on-off" systems.
And in the Wide-Lite bilevel lighting system using metal halide lamps, only 120 VAC control power may be used. Further, such bilevel systems with metal halide lamps need a contactor.
U.S. Pat. Nos. 4,147,962 (Engel); 4,344,701 (Allen) and 4,431,948 (Elder et al.) also depict systems for providing two levels of light output from a lamp. And other systems have emerged, among them a bilevel lighting system shown in U.S. Pat. No. 4,931,701 (Carl). The system depicted in the Carl patent uses a zero crossing relay.
Such earlier systems involve varying degrees of complexity in manufacture and when they are being installed. For example, a system described in trade literature by Day-Brite/Benjamin uses a power line carrier (PLC), a higher-frequency signal superimposed on the 60 Hz AC lines to transmit control signals between a transmitter (which is often coupled with an occupancy detector) and a receiver. In the Day-Brite/Benjamin system, the receiver is on the fixture and there is a phase coupler between the transmitter and receiver to assure that the propagated signal is applied to all three phases of a three-phase system. While PLC systems need no extra control wire, the inclusion of the transmitter, receiver and phase coupler makes then inherently complex.
While there prior art systems have been generally satisfactory for their intended purpose, they have certain disadvantages. To reiterate some of these disadvantages, the PLC system described above requires transmitters, receivers and phase couplers for operation. Other types of bilevel systems require special ballasts configured for the application. Still other types of such systems require plural control wires to effect switching, have certain limitations on control voltage when used with metal halide lamps and require contactors when used with such lamps.
An improved bilevel system overcoming some of these problems and shortcomings would be a distinct advance in the art.